In recent years, increased attention has been directed toward improved energy efficiency in electronic equipment. International standards such as Energy Star provide energy consumption specifications that a product should meet if it is to be certified.
In order to reduce energy consumption, electronic equipment such as printers, typically have a normal mode during which prints can be made, and a sleep mode during which prints cannot be made. In the sleep mode, power is only supplied to certain key portions of the apparatus so that it operates in a low power consumption mode. For example, power is typically provided to a microcontroller in sleep mode so that it is not necessary to reinitialize the firmware when it is time to re-enter normal operating mode. Thus a sleep mode provides energy savings while permitting rapid availability of the printing capability when needed. Even more power savings is possible by turning off the printer entirely, but turning the printer off results in some delay in the availability of printing capability when the printer is turned back on.
Most electronic equipment includes indicator lights, and in many instances, light emitting diodes (LED's) are used. For example, electronic equipment typically includes an LED that serves as a power indicator light that is turned on when the apparatus is on, and is turned off when the apparatus is off. It is desirable to have an indicator light, such as a power indicator LED, which provides a higher amount of light intensity when the apparatus is in normal operating mode and a lower amount of light intensity when the apparatus is in sleep mode.
LED's have a nominal forward voltage drop from anode to cathode when they are providing light at their typical light intensity and the diode current is in the range of around 10 to 20 mA. At a lower forward voltage, the light intensity is reduced or shut off and the current drops off significantly. LED's that provide light in the mid to long wavelength portion of the visible spectrum (such as red, orange, yellow and green) tend to have nominal forward voltages of around 1.6 volts to 2.2 volts. LED's that provide light in the short wavelength portion of the visible spectrum (such as blue) tend to have nominal forward voltages of around 3 volts. A white LED also typically has a nominal forward voltage of around 3 volts, as a white LED is typically made by coating a blue LED with a phosphor of a different color in order to convert some of the emitted light from short wavelengths to longer wavelengths.
Printing systems typically require DC power at a plurality of different voltages. The printing voltage required for the firing pulses for the drop ejectors in an inkjet printhead, for example, is typically between 10 volts and 50 volts depending upon the design of the drop ejectors. Many printheads include driving and logic electronics that is integrated within the same printhead die that includes the drop ejectors. The logic electronics of the printhead requires a DC voltage that is typically around 5 volts. Motor controllers also typically use 5 volts as do the light sources for a document scanner of a multifunction printer. System logic requires a DC voltage that can be around 3.3 volts. Memory, such as DRAM, can require a DC voltage around 2 volts. For systems having an integrated circuit serving as the microcontroller (sometimes called a system on chip or SOC), a core voltage of around 1 V is typically required for the SOC. During normal operating mode, all of these voltages are available. In sleep mode, the 3.3 volt system logic supply is typically left on, but the higher voltages including 5 volts and the printhead drop ejector printing voltage are turned off in order to save energy.
For some printer designs, it is desirable to use a blue LED or a white LED as a power indicator light. Other colors, such as red, green and yellow can unintentionally deliver messages with different connotations due to the usage of such colors in other contexts. A white or blue LED is a more neutral color that does not distract from the meaning that power is on or off.
Consequently, a need exists for a control circuit for an indicator light, such as a white or blue power indicator LED, which provides a higher amount of light intensity when the apparatus is in normal operating mode and a lower amount of light intensity, using a lower amount of energy, when the apparatus is in sleep mode.